1. Field of the Invention
The present invention relates to a multi-polar rotary machine, and more particularly, to a typical hybrid type stepping motor capable of increasing an output and reducing cogging torque for use in an office automation (OA) apparatus or a fully automatic (FA) equipment.
2. Description of the Prior Art
A variable• reluctance (VR) type stepping motor having a rotor using no permanent magnet, a permanent magnet (PM) type stepping motor having a rotor composed of a permanent magnet, and a hybrid (HB) type stepping motor formed by mixing the variable• reluctance type stepping motor and the permanent magnet type stepping motor have been proposed. The permanent magnet type stepping motor and the hybrid type stepping motor are capable of reducing in size and accordingly are used in a relatively small industrial machine. Especially, the hybrid type stepping motor is high in precision and torque and is small in step angle, and accordingly such motor is used widely. However, it is required for the motor to reduce in size and to increase in torque further.
In order to increase a torque, it is effective to increase magnetic flux interlinkaging a winding and a turn number of the winding. A stepping motor capable of increasing the magnetic flux interlinkaging the winding without reducing the resolution or the rotor tooth number is disclosed in the Japanese Patent Application Laid-Open No. 12856/81.
FIG. 26 is a vertically sectioned front view of a hybrid type motor having an outer rotor disclosed in the Japanese Patent Application Laid-Open No. 12856/81. In FIG. 26, a reference numeral 1 denotes a stationary shaft, 2 denotes a front cover, 2′ denotes a rear cover, and 3 denotes a ring shaped magnet magnetized so as to form N and S poles in the axial direction thereof, the magnet 3 being fixed to the stationary shaft 1 passing through a center portion of the magnet 3. Reference numerals 4, 5 denote cylindrical stator elements forming a stator 6 fixed to the shaft 1 corresponding to A phase and B phase, respectively. The stator element 4 has on an outer peripheral surface thereof a plurality of small stator teeth A and Ā separated from each other in the axial direction thereof, the small stator teeth Ā being brought into contact with one side surface of the magnet 3. The stator element 5 has on an outer peripheral surface thereof a plurality of small stator teeth B and B separated from each other in the axial direction thereof, the small stator teeth B being brought into contact with the other side surface of the magnet 3.
A reference numeral 8 denotes an annular groove formed on a peripheral surface at a central portion of the stator element 4, 9 denotes an annular groove formed on a peripheral surface at a central portion of the stator element 5, 12 denotes an exciting winding arranged in the annular groove 8, and 13 denotes an exciting winding arranged in the annular groove 9.
A reference numeral 10 denotes a cylindrical rotor supported rotatably by the shaft 1 through bearings 11 and 11 and covers 2 and 2′. An inner peripheral surface of the rotor 10 faces to an outer peripheral surface of the stator 6 with an air gap therebetween, and has a plurality of small rotor teeth similar in number to the small stator teeth of the stator element 4 or 5.
As shown in FIG. 26A and FIG. 26B, the small stator teeth Ā is circumferentially shifted from the small stator teeth A by 0.5 pitch of the small stator teeth. As shown in FIG. 26C and FIG. 26D, the small stator teeth B is circumferentially shifted from the small stator teeth A by 0.25 pitch of the small stator teeth, and the small stator teeth B is circumferentially shifted from the small stator teeth A by 0.75 pitch of the small stator teeth.
In the above case, the small stator teeth A, B, Ā, and B are circumferentially shifted by 0.25 pitch of the small stator teeth, respectively. In the other case, the small stator teeth are not circumferentially shifted, but the small rotor teeth corresponding to the small stator teeth are circumferentially shifted by 0.25 pitch of the small rotor teeth.
A current flow of the magnetic flux will now be explained. As shown in FIG. 28, a magnetic flux issued from the magnet 3 is entered into the stator element 4, and divided into a magnetic flux φA passing through the small stator teeth A and a magnetic flux φĀ passing though the small stator teeth Ā. The magnetic fluxes φA and φĀ are entered into the rotor 10, directed rightward, and divided into a magnetic flux φB passing through the small stator teeth B and a magnetic flux φ B passing through the small stator teeth B. The magnetic fluxes φB and φ B are directed leftward, and returned to the magnet 3.
The magnetic fluxes φA, φĀ, φB and φ B can be expressed by Formulas 1 to 4, respectively.φA=ΦA(1+k cos θ)  (1)φĀ=ΦĀ(1−k′ cos θ)  (2)φB=ΦB(1+k sin θ)  (3)Φ B=Φ B(1−k′ sin θ)  (4)
Here, θ denotes an electrical angle of the rotation of rotor 10, ΦA, ΦĀ, ΦB and Φ B are mean values of variable magnetic fluxes φA, φĀ, φB and φ B, respectively, and k and k′ are rate of variation. As shown in FIG. 26, the A phase and the B phase are bisymmetric with respect to the magnet except the phase relation, so that ΦA=ΦB, ΦĀ=Φ B and k=k′. Here, it is assumed that the magnetic flux is varied as a sine wave by omtting the harmonic components thereof for the sake of simplicity.
As shown in the Formulas 1 to 4, the magnetic fluxes φA, φB, φĀ and φ B are deviated, respectively, in phase by electrical angle of 90° in this order.
The generated torque is analyzed as follows.
As shown in FIG. 28, effective main magnetic fluxes interlinkaging the windings 12 and 13 for exciting the A phase and the B phase, respectively, are the magnetic fluxes φA and φB, respectively. In case that the rotor 10 is rotated at an electrical angular velocity of ω, counter electromotive forces eA and eB can be expressed by Formulas 5 and 6. Here, n denotes a number of winding of each phase.
                              e          A                =                                            -              n                        ⁢                                          ⅆ                                  ϕ                  A                                                            ⅆ                t                                              =                      n            ⁢                                                  ⁢            Φ            ⁢                                                  ⁢            k            ⁢                                                  ⁢            ω            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                                              (        5        )                                          e          B                =                                            -              n                        ⁢                                          ⅆ                                  ϕ                  B                                                            ⅆ                t                                              =                      n            ⁢                                                  ⁢            Φ            ⁢                                                  ⁢            k            ⁢                                                  ⁢            ω            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                                              (        6        )            
A torque TA and a torque TB can be expressed by Formulas 7 and 8.TA=eAi/ωM=niΦkp sin θ  (7)TB=eBi/ωM=niΦkp cos θ  (8)
Here, ωM denotes a mechanical angular velocity and is ω/p, and p denotes a pole pair number, that is, a number of the small stator teeth or small rotor teeth.
It is appreciated that a mean magnetic flux Φ interlinkaging the winding and a rate of variation k must be increased in order to increase the torque if the number of windings and the number of the small teeth are constant.
In general, each of the rotor and the stator of the motor is formed by laminating a plurality of silicon steel plates, and each silicon steel plate is coated with an anti-corrosion film, so that a gap is formed between laminated silicon steel plates unavoidably. In the conventional motor, many paths of magnetic flux are formed in the axial direction of the motor, and the permeance of iron core is reduced by the gap between the laminated plates, so that the magnetic flux interlinkaging the winding is reduced. Especially, in the motor as shown in FIG. 28, effective main fluxes are only φA and φB and the magnetic fluxes φĀ and φ B are all reactive components. The magnetic flux has a tendency to pass through a magnetic path of smaller magnetic reluctance, so that almost all magnetic fluxes issued from the magnet become φĀ and φ B and the magnetic fluxes φA and φB become very small, and the torque shown in Formulas 5 and 6 is reduced, but the cogging torque is increased, in case of the motor using laminated steel plates.
FIG. 29 shows a waveform of counter electromotive force p of one phase of the conventional motor when it is rotated at 500 revolutions per minute. In this case, the magnitude of the induced voltage is 10V. In this motor, the induced voltage is decreased and the efficiency is lowered compared with the normal motor of similar dimension.
FIG. 30 shows a waveform of cogging torque r of the conventional motor. The magnitude of the cogging torque r is about 0.5 Nm and increased compared with the normal motor of similar dimension, thereby causing a large vibration in the motor when it is rotated.